As a fuel for a fuel cell, hydrogen gas and a liquid and a gaseous fuel of hydrocarbon are used. The fuel cell using a hydrocarbon fuel is classified into a type which reforms the fuel into hydrogen gas by a reformer and generates power with the hydrogen gas as a fuel, and into a type which supplies a hydrocarbon fuel directly into the fuel cell. In the latter fuel cell, the reformer is unnecessary and a whole fuel cell system can be downsized, since the fuel is directly oxidized at a fuel electrode.
The reaction in the fuel cell in which methanol having a high energy density is used, for example, by directly oxidizing the methanol as a hydrocarbon fuel is represented by the formula (1):Fuel Electrode: CH3OH+H2O→CO2+6H++6e−Oxidant Electrode: 3/2O2+6H++e−→3H2OWhole Cell Reaction: CH3OH+3/2O2→CO2+2H2O
As is clear from the formula (1), the volume energy density of 4800 Wh/L is very high, since 1 mol of methanol generates 6 mol of electrons. It is also clear that 1 mol of carbon dioxide is generated as a fuel oxidation product at the fuel electrode in this fuel cell. Based on such generation of carbon dioxide by the power generation, an internal pressure of the fuel cell tends to increase gradually, thereby creating a problem of liquid fuel leakage and deterioration in battery performance.
For such problems, in Japanese Laid-Open Patent Publication No. Hei 10-507572 (pages 17 to 24, FIGS. 1 to 2) and Japanese Laid-Open Patent Publication No. 2001-102070 (pages 1 to 5, FIG. 1), there has been proposed a fuel cell system including a carbon dioxide discharge mechanism in which a porous material of fluorocarbon resin is used to separate carbon dioxide and fuel, and only carbon dioxide is selectively discharged to the outside of the fuel cell, for example.
FIG. 6 is a diagram illustrating a fuel cell system including the carbon dioxide discharge mechanism. In FIG. 6, this type of fuel cell system comprises an electromotive unit 54 of fuel cell, a fuel supply unit 57 for supplying a fuel to the electromotive unit, and an oxidant supply unit 56 for supplying an oxidant to the electromotive unit. Additionally, the fuel supply unit 57 includes a fuel container 51, a pump 52, a fuel supply pipe 53, and a carbon dioxide discharge mechanism 55.
The fuel is supplied from the fuel container 51 by the pump 52 via the fuel supply pipe 53 to the electromotive unit 54 in the direction of an arrow L1, and the oxidant is supplied to the electromotive unit 54 by the oxidant supply unit 56 in the direction of an arrow L2 to generate power. The carbon dioxide produced by the power generation flows in the direction of an arrow L3. The carbon dioxide is discharged to the outside of the fuel cell system in the direction of an arrow L4 by the carbon dioxide discharge mechanism 55 including a film which separates gas and liquid such as a fluorine porous film, for example.
However, the film for separating gas and liquid used as the carbon dioxide discharge mechanism can not completely separate carbon dioxide and fuel selectively, and uses the difference in surface tension of liquid and the separation film to separate gas and liquid. Thus, in addition to carbon dioxide, evaporated fuel and an oxidation product other than the carbon dioxide, and an oxidation by-product (aldehyde and the like, for example) which are produced at the time of the oxidation of fuel are discharged to the outside of the fuel cell as well in gaseous state, passing through the separation film. Therefore, there is a problem in that the fuel and by-product in gaseous state leak out of the fuel cell as vapor.
As a means for solving the problem, it is effective to use a fuel which is difficult to completely combust electrochemically, instead of methanol. Alcohol fuels such as ethanol, propanol, and butanol, for example, are hardly oxidized to become carbon dioxide by complete combustion, even though they are oxidized electrochemically.
In this case, based on the fact that generation of carbon dioxide, which involves enormous volume expansion from liquid to gas, will not occur at the fuel electrode with the oxidation reaction of fuel, the fuel electrode of the fuel cell can be completely sealed. Thus, leakages of evaporated fuel, and fuel oxidation product and by-product to the outside of the fuel cell can be suppressed.
On the other hand, when a fuel other than methanol is used, a very safe fuel cell system without discharging fuel, and fuel oxidation product and by-product can be made. However, since the fuel oxidation reaction itself is incomplete, energy density of fuel will become very low. This point is explained briefly by using propanol and ethylene glycol as examples.
The reaction formula (at the time of complete combustion) of a fuel cell in which propanol is used by direct oxidization is represented by the formula (2):Fuel Electrode: C3H7OH+5H2O→3CO2+18H++18e−Oxidant Electrode: 9/2O2+18H++18e−→9H2OWhole Cell Reaction: C3H7OH+9/2O2→3CO2+4H2OAlso, the reaction formula (at the time of complete combustion) of a fuel cell in which ethylene glycol is used by direct oxidation is represented by formula (3):Fuel Electrode: (CH2OH)2+2H2O→2CO2+10H++10e−Oxidant Electrode: 5/2O2+10H++10e−→5H2OWhole Cell Reaction: (CH2OH)2+5/2O2→2CO2+3H2OIn both propanol and ethylene glycol cases, product will be carbon dioxide when completely combusted, and volume energy densities are respectively very high, showing 7100 Wh/L and 5800 Wh/L.
However, when these fuels are actually used, the reaction at the fuel electrode becomes an incomplete combustion: it is reported that acetone is produced when propanol is used (A. J. Arvia. et al, J. Electroanal. Chem., 350 (1993)97., and Hiromasa Sugii et al., “Summary File of 69th Meeting of Electrochemical Society” (page 79), Apr. 1, 2002, Electrochemical Society of Japan, for example), and that glycolic acid and formic acid are produced when ethylene glycol is used (Takashi Matsuoka et al., “Summary File of 69th Meeting of Electrochemical Society” (page 80) Apr. 1, 2002, Electrochemical Society of Japan, for example).
In these cases, the products are liquid, and have no generation of gas which involves enormous volume expansion: therefore, the fuel electrode can be sealed. However, due to the incomplete combustion of the fuel oxidation reaction, the energy density of fuel is greatly reduced in comparison with the value noted above. For example, when 2-propanol is used as a fuel, with complete combustion, 1 mol of fuel produces 18 mol of electrons, as shown in the formula (2). However, due to an incomplete combustion actually, the fuels are oxidized only to become acetone, producing 2 mol of electrons from 1 mol of fuel.
Therefore, the energy density in this case is extremely low, being 2/18(=790 Wh/L) of the complete combustion. That is, the fuel cell system without having to discharge the fuel, and the fuel oxidation product and by-product had a problem of very low energy density of fuel.
In light of the above problems of the conventional technology, the present invention aims to provide a fuel cell system which can prevent the reduction of energy density due to the cause described above, when the fuel other than methanol which can suppress leakage of the fuel oxidation product and by-product to the outside of the fuel cell is used.